The present invention relates to a digital evidential camera system, a decryption key acquisition/registration system and a digital image editing system.
The photographs and voices recorded in camera films or other media in analog fashion has been conventionally used as admissible evidences in courts and the like. With the recent progress of digital technologies, however, the use of apparatuses for recording images and voices as digital data has extended. These digitized apparatuses have advantages that they are not degenerated even when copied, they can be distributed rapidly using communication lines, and the information contained therein can be readily processed and edited. The ease with which the information can be processed or edited also means the ease with which the information can be altered. This gives rise to the doubt as to the admissibility of such information. For digital images and voices to be used as admissible information, therefore, some method having the function of preventing the alteration of digital data is required. A camera having such a protective function is called a digital evidential camera.
In order to realize this digital evidential camera, an application of the digital signature technique generally used in communications or the like has been conceived. The digital signature system uses a pair of keys. One is called a private key for encryption, and the other a public key for decryption. Digital data are encrypted using the private key, and decrypted using the public key. The pair of a public key and a private key have a characteristic of being very difficult to mathematically obtain a private key or a public key from a public key or a private key respectively. The private key is required to be strictly managed in such a manner as never to be used by other than the owner, while the public key is made public for general use.
In a method of detecting an alteration, a first step is to extract a code of predetermined size called message digest (hereinafter called MD) at the transmitting end, using the digest algorithm based on a hash function from the digital data involved. A method of extracting the MD from the digital data involved is made public, and any person having the original data can extract the MD. The MD has such a characteristic based on the well-known characteristic of the hash function as to undergo a considerable change with a small change in the original digital data.
The MD thus extracted is encrypted using the private key. With this as a message authentication code (hereinafter called the MAC), the encrypted MD is transmitted to the other party together with the original data. It is assumed that the public key paired with the private key has been delivered without fail to the receiving party (the receiving party is required to have acquired the key without fail, and may be delivered to a third party).
Recently, an alteration prevention code using the public key encryption algorithm is called the Digital Signature and an alteration prevention code using the common key encryption algorithm is called the Message Authentication Code (MAC). However, in this specification, a MAC is defined broadly as those of including an alteration prevention code using the common key encryption algorithm and an alteration prevention code using the private key/public key encryption algorithm.
At the receiving end, in order to check that the original data is not altered, first determines the MD′ using the message digest algorithm from the original data. Then, using the public key, he decrypts the MAC to determine the MD, and checks whether this MD coincides with the MD′ or not. Even if the original data is altered by a third party, the third party who owns no private key cannot produce the MAC that can be decrypted with the public key. Therefore, the MD and the MD′ assume different values. As a result, it can be found that the original data has been altered by a third party.
As described above, the digital signature technique can be used for detecting the alteration of digital data. In the case where the method of detection of an alteration described above is used for a digital evidential camera, the private key as an encryption key must never leak outside. In the prior art, however, it is not an easy matter to manage the private key at a high security level, and therefore the admissibility of the digital image cannot be improved.
Image data, for their characteristics, are often required to be compressed, cut out in areas, have a caption inserted therein or otherwise processed. According to a conventional method of digital signature used for document data, a slight change in data contents is regarded as an alteration of the data. With the conventional digital signature system, therefore, the required editing work can never be accomplished due to the above-mentioned characteristics of the image data.
Accordingly, the object of the present invention is to provide a digital evidential camera system and a decryption key acquisition/registration system which can be managed at a very high security level, and a digital image editing system which can maintain the admissibility of a digital image even after it is compressed, cut out in areas, has a caption inserted therein or otherwise edited as required for its characteristics.